3，With some children, small sums go a long way.

 这句话暗含着：With other children, small sums can’t go a long way.

go a long way 维持很久

e.g. The money we have will go a long way.

go a long way towards: 对……大有帮助

e.g. This will go a long way towards overcoming the difficulty.(overcome: 战胜，克服)

介词With和For的区别：“对于……来说”

(1)介词for----“关于，对于……来说，考虑到……的事实”

e.g. The weather is quite warm for November. 对十一月来说，这儿的天气很暖和了。

e.g. For him, this will be an entirely new hobby. 对他来说，这是全新的爱好。

(2)介词with----“在某一方面”（多用于二者的比较）

e.g. With some people, pleasure is more important than work. 对有些人来说，享乐比工作重要。

e.g. Some people may accept that excuse, but it won’t work with me. 有些人可能会接受那样的借口，但是我绝对不容许。

4, If fifty pence pieces are not exchanged for sweets, they rattle for months inside money boxes.

piece 这里是硬币的意思，等于：coin

pence: 便士 （penny 便士的复数：pence）

  a fifty pence: 一个五十便士的硬币

  a fifty penies: 五十个一便士的硬币新概念第三册

exchanges for: 换取，以......来交换

e.g. The little girl exchanged her pen for candy.